Ask the six-term senator, and you get the sort of aw-shucks response that has endeared him to Iowans — who refer to him simply as "Grassley" at the state fair — for so long.

“The best way to get re-elected is doing the very best job you can as an office holder and you’ll be retained in office, or the chances of being retained in office are better,” Grassley told CQ Roll Call in a phone interview Wednesday after leaving a town hall in Albia, part of his famed annual tour of each of Iowa's 99 counties. (Grassley is adamant campaign events don't go toward the count.)

That work ethic is just one reason Republicans and Democrats alike aren't putting Grassley's race in the Tossup category.

Former aides and Iowa Republicans cite a simple formula for Grassley’s victories.

“He just goes home, he’s in touch, he’s the same person. I mean it’s really as simple as that,” said freshman Rep. David Young, Grassley’s former chief of staff. “There’s no secret underground blueprint.”

The 99-county tour has earned the nickname ‘The Full Grassley," and it's copied by White House hopefuls and the state’s junior senator, Joni Ernst. (He may not mind she's borrowed it — she told CQ Roll Call that Grassley drops by her constituent coffees on Capitol Hill to make sure he didn't miss anyone.)

As the senator makes his way across the state, he seems to know just about everyone.

“People will come up to him and say, ‘You know, we met once before,’ and he’ll say, ‘Oh yeah, I know, it was 1986 and I was sitting next to you on an airplane, we talked about your mom’s upholstery shop,’” former aide Cory Crowley said.

Mike Steenhoek, once Grassley's scheduler, said aides need to frequently pad his schedule with extra time to account for such conversations.

At some town halls, the senator “gets out of the car and he runs into the building just because he might be a few minutes late. He just thinks it’s a disrespect to people’s time to be late,” Steenhoek said.

Once there, he listens to people’s questions, even when they disagree with him, and takes the time to explain his position. The approach has earned him the respect of people who do not always like how he votes or what he does.

“Maybe people disagree with him, but they think that he’s doing what he thinks is right,” Republican Gov. Terry Branstad told CQ Roll Call.

Grassley takes the same approach to constituent services back in Washington, insisting every letter "had to be answered and had to be answered in full,” said Dean Zerbe, a former Finance Committee staffer who worked closely with Grassley. Usually that means a 2- or 3-page letter on the bill's history and why Grassley has that particular position.

The senator meets with his legislative correspondents “at least once a week” to help craft responses, find out what people are writing about, and if there are any “themes.” If response time starts to lag, “that really stresses him out,” Crowley said.

Democrats looking to topple him will not have an easy path. His lowest re-election margin of victory was 64 percent in 2010. (He won 66 percent of the vote in 1986, 70 percent in 1992, 68 percent in 1998, and 70 percent in 2004, all in a state where Republicans comprise just under one third of registered voters.) Those would be strong percentages even for a Republican in a favorable state. Sen. John Cornyn won a virtually uncontested re-election in heavily RepublicanTexas with just 62 percent of the vote last fall. In Oklahoma, GOP Sen. Jim Inhofe won with 68 percent.

But even 70 percent doesn't satisfy the senator. Crowley said Grassley will ask, in all seriousness, "Can we find out who the 30 percent of people who didn’t vote for me are so over the next six years we can call them?"

Such long tenure isn't all that unusual in Iowa. Branstad is in his 6th non-consecutive term, on his way to becoming the longest-serving governor in U.S. history. Democratic Sen. Tom Harkin served five terms in the Senate before retiring, though he had never won re-election with more than 55 percent of the vote until his 63 percent victory in 2008.

Iowa Republicans said Grassley earned voters’ respect by using his committee perches to go after wasted money in the spending of administrations both Republican and Democrat.

“He’s an equal opportunity critiquer on both sides of the aisle,” Young said. “He’s gone after this president, but he’s gone after Ronald Reagan as president, you know his Defense Department.”

He has not missed a Senate vote since 1993, a record unrivaled by his colleagues. But, multiple former staffers said, at the end of the week, he just wants to go home.

“He is so keen to get back to Iowa, come Thursday afternoon he’s like a pacing cat wanting to get back on that plane and get back to Iowa,” Zerbe said.

Democrats acknowledge Grassley would be tough to beat, but pledge they'll find a good candidate.

“Sen. Grassley has a long record that he’ll have to defend,” said Democratic Senatorial Campaign Committee communications director Justin Barasky.

The race is rated Favored Republican by the Rothenberg & Gonzales Political Report/Roll Call. But presidential year turnout could boost Democratic hopes.

“I would suspect this is the worst environment he’s going to have faced in a long time ... It might be a sensible time to take a shot at the guy,” said Iowa Democratic consultant Travis Lowe.

Grassley has usually stayed out of the presidential caucuses, and plans to do so this year. And while he said he would “encourage” candidates to come Iowa, he has been known to skip events with presidential hopefuls in favor of other pursuits.

Branstad told CQ Roll Call that he and Grassley attended the high school girls basketball state championship tournament in March. It was the night before the Iowa Ag Summit, an event attended by a number of the Republican presidential hopefuls. Before the tournament was over, Branstad excused himself so he could meet the candidates at a reception.

Branstad said the senator wasn't budging from the bleachers: "Chuck Grassley said, ‘Will you give them my regrets? Because I haven’t missed a basketball tournament in 36 years.'"

Correction 11:15 a.m. A previous version of this story misquoted Grassley on the number of years that have elapsed since he's missed a basketball tournament. It's 36 years.